diff --git a/Cargo.lock b/Cargo.lock
index f9b3a03dc..af7d471ad 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -489,6 +489,7 @@ dependencies = [
  "image-meta 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "imap 1.0.2 (git+https://github.com/jonhoo/rust-imap?rev=281d2eb8ab50dc656ceff2ae749ca5045f334e15)",
  "itertools 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "jetscii 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "lettre 0.9.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.62 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1010,6 +1011,11 @@ name = "itoa"
 version = "0.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "jetscii"
+version = "0.4.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+
 [[package]]
 name = "keccak"
 version = "0.1.0"
@@ -2960,6 +2966,7 @@ dependencies = [
 "checksum iovec 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)" = "dbe6e417e7d0975db6512b90796e8ce223145ac4e33c377e4a42882a0e88bb08"
 "checksum itertools 0.8.0 (registry+https://github.com/rust-lang/crates.io-index)" = "5b8467d9c1cebe26feb08c640139247fac215782d35371ade9a2136ed6085358"
 "checksum itoa 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)" = "501266b7edd0174f8530248f87f99c88fbe60ca4ef3dd486835b8d8d53136f7f"
+"checksum jetscii 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)" = "5f25cca2463cb19dbb1061eb3bd38a8b5e4ce1cc5a5a9fc0e02de486d92b9b05"
 "checksum keccak 0.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "67c21572b4949434e4fc1e1978b99c5f77064153c59d998bf13ecd96fb5ecba7"
 "checksum kernel32-sys 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "7507624b29483431c0ba2d82aece8ca6cdba9382bff4ddd0f7490560c056098d"
 "checksum lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)" = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
diff --git a/Cargo.toml b/Cargo.toml
index 7cb3cd88b..b992f8874 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -50,6 +50,7 @@ image-meta = "0.1.0"
 quick-xml = "0.15.0"
 escaper = "0.1.0"
 bitflags = "1.1.0"
+jetscii = "0.4.4"
 
 [dev-dependencies]
 tempfile = "3.0"
diff --git a/proptest-regressions/dc_strencode.txt b/proptest-regressions/dc_strencode.txt
new file mode 100644
index 000000000..3ef09f145
--- /dev/null
+++ b/proptest-regressions/dc_strencode.txt
@@ -0,0 +1,7 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 679506fe9ac59df773f8cfa800fdab5f0a32fe49d2ab370394000a1aa5bc2a72 # shrinks to buf = "%0A"
diff --git a/src/dc_mimefactory.rs b/src/dc_mimefactory.rs
index e15a3a5e9..e5611779e 100644
--- a/src/dc_mimefactory.rs
+++ b/src/dc_mimefactory.rs
@@ -1218,7 +1218,7 @@ unsafe fn build_body_file(
             /* create mime part, for Content-Disposition, see RFC 2183.
             `Content-Disposition: attachment` seems not to make a difference to `Content-Disposition: inline` at least on tested Thunderbird and Gma'l in 2017.
             But I've heard about problems with inline and outl'k, so we just use the attachment-type until we run into other problems ... */
-            needs_ext = dc_needs_ext_header(filename_to_send);
+            needs_ext = dc_needs_ext_header(as_str(filename_to_send));
             mime_fields = mailmime_fields_new_filename(
                 MAILMIME_DISPOSITION_TYPE_ATTACHMENT as libc::c_int,
                 if needs_ext {
@@ -1255,7 +1255,7 @@ unsafe fn build_body_file(
                                         strdup(
                                             b"filename*\x00" as *const u8 as *const libc::c_char,
                                         ),
-                                        dc_encode_ext_header(filename_to_send),
+                                        dc_encode_ext_header(as_str(filename_to_send)).strdup(),
                                     ),
                                 );
                             if !parm.is_null() {
diff --git a/src/dc_mimeparser.rs b/src/dc_mimeparser.rs
index cf99d24d2..5d68547fc 100644
--- a/src/dc_mimeparser.rs
+++ b/src/dc_mimeparser.rs
@@ -1204,8 +1204,8 @@ unsafe fn dc_mimeparser_add_single_part_if_known(
                         }
                         if !filename_parts.is_empty() {
                             free(desired_filename as *mut libc::c_void);
-                            let parts_c = CString::yolo(filename_parts);
-                            desired_filename = dc_decode_ext_header(parts_c.as_ptr());
+                            desired_filename =
+                                dc_decode_ext_header(filename_parts.as_bytes()).strdup();
                         }
                         if desired_filename.is_null() {
                             let param = mailmime_find_ct_parameter(
diff --git a/src/dc_strencode.rs b/src/dc_strencode.rs
index 8e0ffcf39..e1f4d1526 100644
--- a/src/dc_strencode.rs
+++ b/src/dc_strencode.rs
@@ -1,83 +1,17 @@
-use std::ffi::{CStr, CString};
+use std::borrow::Cow;
+use std::ffi::CString;
 use std::ptr;
 
 use charset::Charset;
 use mmime::mailmime_decode::*;
 use mmime::mmapstring::*;
 use mmime::other::*;
+use percent_encoding::{percent_decode, utf8_percent_encode, AsciiSet, CONTROLS};
 
 use crate::dc_tools::*;
 use crate::types::*;
 use crate::x::*;
 
-#[inline]
-fn isalnum(c: libc::c_int) -> libc::c_int {
-    if c < std::u8::MAX as libc::c_int {
-        (c as u8 as char).is_ascii_alphanumeric() as libc::c_int
-    } else {
-        0
-    }
-}
-
-/* ******************************************************************************
- * URL encoding and decoding, RFC 3986
- ******************************************************************************/
-unsafe fn int_2_uppercase_hex(code: libc::c_char) -> libc::c_char {
-    static mut HEX: [libc::c_char; 17] = [
-        48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 65, 66, 67, 68, 69, 70, 0,
-    ];
-
-    HEX[(code as libc::c_int & 15i32) as usize]
-}
-
-pub unsafe fn dc_urldecode(to_decode: *const libc::c_char) -> *mut libc::c_char {
-    let mut pstr: *const libc::c_char = to_decode;
-    if to_decode.is_null() {
-        return dc_strdup(b"\x00" as *const u8 as *const libc::c_char);
-    }
-    let buf: *mut libc::c_char = malloc(strlen(to_decode).wrapping_add(1)) as *mut libc::c_char;
-    let mut pbuf: *mut libc::c_char = buf;
-    assert!(!buf.is_null());
-
-    while 0 != *pstr {
-        if *pstr as libc::c_int == '%' as i32 {
-            if 0 != *pstr.offset(1isize) as libc::c_int && 0 != *pstr.offset(2isize) as libc::c_int
-            {
-                let fresh5 = pbuf;
-                pbuf = pbuf.offset(1);
-                *fresh5 = ((hex_2_int(*pstr.offset(1isize)) as libc::c_int) << 4i32
-                    | hex_2_int(*pstr.offset(2isize)) as libc::c_int)
-                    as libc::c_char;
-                pstr = pstr.offset(2isize)
-            }
-        } else if *pstr as libc::c_int == '+' as i32 {
-            let fresh6 = pbuf;
-            pbuf = pbuf.offset(1);
-            *fresh6 = ' ' as i32 as libc::c_char
-        } else {
-            let fresh7 = pbuf;
-            pbuf = pbuf.offset(1);
-            *fresh7 = *pstr
-        }
-        pstr = pstr.offset(1isize)
-    }
-    *pbuf = '\u{0}' as i32 as libc::c_char;
-
-    buf
-}
-
-fn hex_2_int(ch: libc::c_char) -> libc::c_char {
-    let ch = ch as u8 as char;
-    if !ch.is_ascii_hexdigit() {
-        return (ch.to_ascii_lowercase() as i32 - 'a' as i32 + 10) as libc::c_char;
-    }
-
-    match ch.to_digit(16) {
-        Some(res) => res as libc::c_char,
-        None => 0,
-    }
-}
-
 pub unsafe fn dc_encode_header_words(to_encode: *const libc::c_char) -> *mut libc::c_char {
     let mut ok_to_continue = true;
     let mut ret_str: *mut libc::c_char = ptr::null_mut();
@@ -290,374 +224,63 @@ pub unsafe fn dc_decode_header_words(in_0: *const libc::c_char) -> *mut libc::c_
     out
 }
 
-#[cfg(test)]
-unsafe fn dc_encode_modified_utf7(
-    mut to_encode: *const libc::c_char,
-    change_spaces: libc::c_int,
-) -> *mut libc::c_char {
-    let mut utf8pos: libc::c_uint;
-    let mut utf8total: libc::c_uint;
-    let mut c: libc::c_uint;
-    let mut utf7mode: libc::c_uint;
-    let mut bitstogo: libc::c_uint;
-    let mut utf16flag: libc::c_uint;
-    let mut ucs4: libc::c_ulong = 0;
-    let mut bitbuf: libc::c_ulong = 0;
-    let mut dst: *mut libc::c_char;
-    let res: *mut libc::c_char;
-    if to_encode.is_null() {
-        return dc_strdup(b"\x00" as *const u8 as *const libc::c_char);
-    }
-    res = malloc(2usize.wrapping_mul(strlen(to_encode)).wrapping_add(1)) as *mut libc::c_char;
-    dst = res;
-    assert!(!dst.is_null());
+pub fn dc_needs_ext_header(to_check: impl AsRef<str>) -> bool {
+    let to_check = to_check.as_ref();
 
-    utf7mode = 0i32 as libc::c_uint;
-    utf8total = 0i32 as libc::c_uint;
-    bitstogo = 0i32 as libc::c_uint;
-    utf8pos = 0i32 as libc::c_uint;
-    loop {
-        c = *to_encode as libc::c_uchar as libc::c_uint;
-        if !(c != '\u{0}' as i32 as libc::c_uint) {
-            break;
-        }
-        to_encode = to_encode.offset(1isize);
-        // normal character?
-        if c >= ' ' as i32 as libc::c_uint
-            && c <= '~' as i32 as libc::c_uint
-            && (c != '_' as i32 as libc::c_uint || 0 == change_spaces)
-        {
-            if 0 != utf7mode {
-                if 0 != bitstogo {
-                    let fresh8 = dst;
-                    dst = dst.offset(1);
-                    *fresh8 = BASE64CHARS
-                        [(bitbuf << (6i32 as libc::c_uint).wrapping_sub(bitstogo) & 0x3f) as usize]
-                }
-                let fresh9 = dst;
-                dst = dst.offset(1);
-                *fresh9 = '-' as i32 as libc::c_char;
-                utf7mode = 0i32 as libc::c_uint;
-                utf8pos = 0i32 as libc::c_uint;
-                bitstogo = 0i32 as libc::c_uint;
-                utf8total = 0i32 as libc::c_uint
-            }
-            if 0 != change_spaces && c == ' ' as i32 as libc::c_uint {
-                let fresh10 = dst;
-                dst = dst.offset(1);
-                *fresh10 = '_' as i32 as libc::c_char
-            } else {
-                let fresh11 = dst;
-                dst = dst.offset(1);
-                *fresh11 = c as libc::c_char
-            }
-            if c == '&' as i32 as libc::c_uint {
-                let fresh12 = dst;
-                dst = dst.offset(1);
-                *fresh12 = '-' as i32 as libc::c_char
-            }
-        } else {
-            if 0 == utf7mode {
-                let fresh13 = dst;
-                dst = dst.offset(1);
-                *fresh13 = '&' as i32 as libc::c_char;
-                utf7mode = 1i32 as libc::c_uint
-            }
-            // encode ascii characters as themselves
-            if c < 0x80i32 as libc::c_uint {
-                ucs4 = c as libc::c_ulong
-            } else if 0 != utf8total {
-                ucs4 = ucs4 << 6i32 | c as libc::c_ulong & 0x3f;
-                utf8pos = utf8pos.wrapping_add(1);
-                if utf8pos < utf8total {
-                    continue;
-                }
-            } else {
-                utf8pos = 1i32 as libc::c_uint;
-                if c < 0xe0i32 as libc::c_uint {
-                    utf8total = 2i32 as libc::c_uint;
-                    ucs4 = (c & 0x1fi32 as libc::c_uint) as libc::c_ulong
-                } else if c < 0xf0i32 as libc::c_uint {
-                    utf8total = 3i32 as libc::c_uint;
-                    ucs4 = (c & 0xfi32 as libc::c_uint) as libc::c_ulong
-                } else {
-                    utf8total = 4i32 as libc::c_uint;
-                    ucs4 = (c & 0x3i32 as libc::c_uint) as libc::c_ulong
-                }
-                continue;
-            }
-            utf8total = 0i32 as libc::c_uint;
-            loop {
-                if ucs4 >= 0x10000 {
-                    ucs4 = ucs4.wrapping_sub(0x10000);
-                    bitbuf = bitbuf << 16 | (ucs4 >> 10).wrapping_add(0xd800);
-                    ucs4 = (ucs4 & 0x3ff).wrapping_add(0xdc00);
-                    utf16flag = 1i32 as libc::c_uint
-                } else {
-                    bitbuf = bitbuf << 16 | ucs4;
-                    utf16flag = 0i32 as libc::c_uint
-                }
-                bitstogo = bitstogo.wrapping_add(16i32 as libc::c_uint);
-                while bitstogo >= 6i32 as libc::c_uint {
-                    bitstogo = bitstogo.wrapping_sub(6i32 as libc::c_uint);
-                    let fresh14 = dst;
-                    dst = dst.offset(1);
-                    *fresh14 = BASE64CHARS[(if 0 != bitstogo {
-                        bitbuf >> bitstogo
-                    } else {
-                        bitbuf
-                    } & 0x3f) as usize]
-                }
-                if !(0 != utf16flag) {
-                    break;
-                }
-            }
-        }
+    if to_check.is_empty() {
+        return false;
     }
-    if 0 != utf7mode {
-        if 0 != bitstogo {
-            let fresh15 = dst;
-            dst = dst.offset(1);
-            *fresh15 = BASE64CHARS
-                [(bitbuf << (6i32 as libc::c_uint).wrapping_sub(bitstogo) & 0x3f) as usize]
-        }
-        let fresh16 = dst;
-        dst = dst.offset(1);
-        *fresh16 = '-' as i32 as libc::c_char
-    }
-    *dst = '\u{0}' as i32 as libc::c_char;
 
-    res
+    to_check.chars().any(|c| {
+        !(c.is_ascii_alphanumeric()
+            || c == '-'
+            || c == '_'
+            || c == '_'
+            || c == '.'
+            || c == '~'
+            || c == '%')
+    })
 }
 
-/* ******************************************************************************
- * Encode/decode modified UTF-7 as needed for IMAP, see RFC 2192
- ******************************************************************************/
+const EXT_ASCII_ST: &AsciiSet = &CONTROLS
+    .add(b' ')
+    .add(b'-')
+    .add(b'_')
+    .add(b'.')
+    .add(b'~')
+    .add(b'%');
 
-// UTF7 modified base64 alphabet
-#[cfg(test)]
-static mut BASE64CHARS: [libc::c_char; 65] = [
-    65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88,
-    89, 90, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114,
-    115, 116, 117, 118, 119, 120, 121, 122, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 43, 44, 0,
-];
-
-#[cfg(test)]
-unsafe fn dc_decode_modified_utf7(
-    to_decode: *const libc::c_char,
-    change_spaces: libc::c_int,
-) -> *mut libc::c_char {
-    let mut c: libc::c_uint;
-    let mut i: libc::c_uint;
-    let mut bitcount: libc::c_uint;
-    let mut ucs4: libc::c_ulong;
-    let mut utf16: libc::c_ulong;
-    let mut bitbuf: libc::c_ulong;
-    let mut base64: [libc::c_uchar; 256] = [0; 256];
-    let mut src: *const libc::c_char;
-    let mut dst: *mut libc::c_char;
-    let res: *mut libc::c_char;
-    if to_decode.is_null() {
-        return dc_strdup(b"\x00" as *const u8 as *const libc::c_char);
-    }
-    res = malloc(4usize.wrapping_mul(strlen(to_decode)).wrapping_add(1)) as *mut libc::c_char;
-    dst = res;
-    src = to_decode;
-    assert!(!dst.is_null());
-
-    libc::memset(
-        base64.as_mut_ptr() as *mut libc::c_void,
-        64,
-        ::std::mem::size_of::<[libc::c_uchar; 256]>(),
-    );
-    i = 0i32 as libc::c_uint;
-    while (i as libc::c_ulong) < ::std::mem::size_of::<[libc::c_char; 65]>() as libc::c_ulong {
-        base64[BASE64CHARS[i as usize] as libc::c_uint as usize] = i as libc::c_uchar;
-        i = i.wrapping_add(1)
-    }
-    while *src as libc::c_int != '\u{0}' as i32 {
-        let fresh17 = src;
-        src = src.offset(1);
-        c = *fresh17 as libc::c_uint;
-        if c != '&' as i32 as libc::c_uint || *src as libc::c_int == '-' as i32 {
-            if 0 != change_spaces && c == '_' as i32 as libc::c_uint {
-                let fresh18 = dst;
-                dst = dst.offset(1);
-                *fresh18 = ' ' as i32 as libc::c_char
-            } else {
-                let fresh19 = dst;
-                dst = dst.offset(1);
-                *fresh19 = c as libc::c_char
-            }
-            if c == '&' as i32 as libc::c_uint {
-                src = src.offset(1isize)
-            }
-        } else {
-            bitbuf = 0;
-            bitcount = 0i32 as libc::c_uint;
-            ucs4 = 0;
-            loop {
-                c = base64[*src as libc::c_uchar as usize] as libc::c_uint;
-                if !(c != 64i32 as libc::c_uint) {
-                    break;
-                }
-                src = src.offset(1isize);
-                bitbuf = bitbuf << 6i32 | c as libc::c_ulong;
-                bitcount = bitcount.wrapping_add(6i32 as libc::c_uint);
-                // enough bits for a UTF-16 character?
-                if !(bitcount >= 16i32 as libc::c_uint) {
-                    continue;
-                }
-                bitcount = bitcount.wrapping_sub(16i32 as libc::c_uint);
-                utf16 = if 0 != bitcount {
-                    bitbuf >> bitcount
-                } else {
-                    bitbuf
-                } & 0xffff;
-
-                // convert UTF16 to UCS4
-                if utf16 >= 0xd800 && utf16 <= 0xdbff {
-                    ucs4 = utf16.wrapping_sub(0xd800) << 10i32
-                } else {
-                    if utf16 >= 0xdc00 && utf16 <= 0xdfff {
-                        ucs4 = ucs4.wrapping_add(utf16.wrapping_sub(0xdc00).wrapping_add(0x10000))
-                    } else {
-                        ucs4 = utf16
-                    }
-                    if ucs4 <= 0x7f {
-                        *dst.offset(0isize) = ucs4 as libc::c_char;
-                        dst = dst.offset(1isize)
-                    } else if ucs4 <= 0x7ff {
-                        *dst.offset(0isize) = (0xc0 | ucs4 >> 6i32) as libc::c_char;
-                        *dst.offset(1isize) = (0x80 | ucs4 & 0x3f) as libc::c_char;
-                        dst = dst.offset(2isize)
-                    } else if ucs4 <= 0xffff {
-                        *dst.offset(0isize) = (0xe0 | ucs4 >> 12i32) as libc::c_char;
-                        *dst.offset(1isize) = (0x80 | ucs4 >> 6i32 & 0x3f) as libc::c_char;
-                        *dst.offset(2isize) = (0x80 | ucs4 & 0x3f) as libc::c_char;
-                        dst = dst.offset(3isize)
-                    } else {
-                        *dst.offset(0isize) = (0xf0 | ucs4 >> 18i32) as libc::c_char;
-                        *dst.offset(1isize) = (0x80 | ucs4 >> 12i32 & 0x3f) as libc::c_char;
-                        *dst.offset(2isize) = (0x80 | ucs4 >> 6i32 & 0x3f) as libc::c_char;
-                        *dst.offset(3isize) = (0x80 | ucs4 & 0x3f) as libc::c_char;
-                        dst = dst.offset(4isize)
-                    }
-                }
-            }
-            if *src as libc::c_int == '-' as i32 {
-                src = src.offset(1isize)
-            }
-        }
-    }
-    *dst = '\u{0}' as i32 as libc::c_char;
-
-    res
+/// Encode an UTF-8 string to the extended header format.
+pub fn dc_encode_ext_header(to_encode: impl AsRef<str>) -> String {
+    let encoded = utf8_percent_encode(to_encode.as_ref(), &EXT_ASCII_ST);
+    format!("utf-8''{}", encoded)
 }
 
-pub unsafe fn dc_needs_ext_header(mut to_check: *const libc::c_char) -> bool {
-    if !to_check.is_null() {
-        while 0 != *to_check {
-            if 0 == isalnum(*to_check as libc::c_int)
-                && *to_check as libc::c_int != '-' as i32
-                && *to_check as libc::c_int != '_' as i32
-                && *to_check as libc::c_int != '.' as i32
-                && *to_check as libc::c_int != '~' as i32
-            {
-                return true;
-            }
-            to_check = to_check.offset(1isize)
-        }
-    }
-
-    false
-}
-
-pub unsafe fn dc_encode_ext_header(to_encode: *const libc::c_char) -> *mut libc::c_char {
-    let mut pstr: *const libc::c_char = to_encode;
-    if to_encode.is_null() {
-        return dc_strdup(b"utf-8\'\'\x00" as *const u8 as *const libc::c_char);
-    }
-    let buf: *mut libc::c_char = malloc(
-        strlen(b"utf-8\'\'\x00" as *const u8 as *const libc::c_char)
-            .wrapping_add(strlen(to_encode).wrapping_mul(3))
-            .wrapping_add(1),
-    ) as *mut libc::c_char;
-    assert!(!buf.is_null());
-
-    let mut pbuf: *mut libc::c_char = buf;
-    strcpy(pbuf, b"utf-8\'\'\x00" as *const u8 as *const libc::c_char);
-    pbuf = pbuf.offset(strlen(pbuf) as isize);
-    while 0 != *pstr {
-        if 0 != isalnum(*pstr as libc::c_int)
-            || *pstr as libc::c_int == '-' as i32
-            || *pstr as libc::c_int == '_' as i32
-            || *pstr as libc::c_int == '.' as i32
-            || *pstr as libc::c_int == '~' as i32
-        {
-            let fresh20 = pbuf;
-            pbuf = pbuf.offset(1);
-            *fresh20 = *pstr
-        } else {
-            let fresh21 = pbuf;
-            pbuf = pbuf.offset(1);
-            *fresh21 = '%' as i32 as libc::c_char;
-            let fresh22 = pbuf;
-            pbuf = pbuf.offset(1);
-            *fresh22 = int_2_uppercase_hex((*pstr as libc::c_int >> 4i32) as libc::c_char);
-            let fresh23 = pbuf;
-            pbuf = pbuf.offset(1);
-            *fresh23 = int_2_uppercase_hex((*pstr as libc::c_int & 15i32) as libc::c_char)
-        }
-        pstr = pstr.offset(1isize)
-    }
-    *pbuf = '\u{0}' as i32 as libc::c_char;
-
-    buf
-}
-
-pub unsafe fn dc_decode_ext_header(to_decode: *const libc::c_char) -> *mut libc::c_char {
-    let mut decoded: *mut libc::c_char = ptr::null_mut();
-    let mut charset: *mut libc::c_char = ptr::null_mut();
-    let mut p2: *const libc::c_char;
-    if !to_decode.is_null() {
-        // get char set
-        p2 = strchr(to_decode, '\'' as i32);
-        if !(p2.is_null() || p2 == to_decode) {
-            /*no empty charset allowed*/
-            charset =
-                dc_null_terminate(to_decode, p2.wrapping_offset_from(to_decode) as libc::c_int);
-            p2 = p2.offset(1isize);
+/// Decode an extended-header-format strings to UTF-8.
+pub fn dc_decode_ext_header(to_decode: &[u8]) -> Cow<str> {
+    if let Some(index) = bytes!(b'\'').find(to_decode) {
+        let (charset, rest) = to_decode.split_at(index);
+        if !charset.is_empty() {
             // skip language
-            p2 = strchr(p2, '\'' as i32);
-            if !p2.is_null() {
-                p2 = p2.offset(1isize);
-                decoded = dc_urldecode(p2);
-                if !charset.is_null()
-                    && strcmp(charset, b"utf-8\x00" as *const u8 as *const libc::c_char) != 0i32
-                    && strcmp(charset, b"UTF-8\x00" as *const u8 as *const libc::c_char) != 0i32
-                {
-                    if let Some(encoding) =
-                        Charset::for_label(CStr::from_ptr(charset).to_str().unwrap().as_bytes())
-                    {
-                        let data =
-                            std::slice::from_raw_parts(decoded as *const u8, strlen(decoded));
+            if let Some(index2) = bytes!(b'\'').find(&rest[1..]) {
+                let decoded = percent_decode(&rest[index2 + 2..]);
 
-                        let (res, _, _) = encoding.decode(data);
-                        free(decoded as *mut _);
-                        let r = std::ffi::CString::new(res.as_bytes()).unwrap();
-                        decoded = dc_strdup(r.as_ptr());
+                if charset != b"utf-8" && charset != b"UTF-8" {
+                    if let Some(encoding) = Charset::for_label(charset) {
+                        let bytes = decoded.collect::<Vec<u8>>();
+                        let (res, _, _) = encoding.decode(&bytes);
+                        return Cow::Owned(res.into_owned());
+                    } else {
+                        return decoded.decode_utf8_lossy();
                     }
+                } else {
+                    return decoded.decode_utf8_lossy();
                 }
             }
         }
     }
-    free(charset as *mut libc::c_void);
-    if !decoded.is_null() {
-        decoded
-    } else {
-        dc_strdup(to_decode)
-    }
+
+    String::from_utf8_lossy(to_decode)
 }
 
 unsafe fn print_hex(target: *mut libc::c_char, cur: *const libc::c_char) {
@@ -672,16 +295,8 @@ unsafe fn print_hex(target: *mut libc::c_char, cur: *const libc::c_char) {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use percent_encoding::{percent_encode, NON_ALPHANUMERIC};
     use std::ffi::CStr;
 
-    #[test]
-    fn test_isalnum() {
-        assert_eq!(isalnum(0), 0);
-        assert_eq!(isalnum('5' as libc::c_int), 1);
-        assert_eq!(isalnum('Q' as libc::c_int), 1);
-    }
-
     #[test]
     fn test_dc_decode_header_words() {
         unsafe {
@@ -742,99 +357,43 @@ mod tests {
 
     #[test]
     fn test_dc_encode_ext_header() {
-        unsafe {
-            let mut buf1 = dc_encode_ext_header(
-                b"Bj\xc3\xb6rn Petersen\x00" as *const u8 as *const libc::c_char,
-            );
-            assert_eq!(
-                CStr::from_ptr(buf1).to_str().unwrap(),
-                "utf-8\'\'Bj%C3%B6rn%20Petersen"
-            );
-            let buf2 = dc_decode_ext_header(buf1);
-            assert_eq!(
-                strcmp(
-                    buf2,
-                    b"Bj\xc3\xb6rn Petersen\x00" as *const u8 as *const libc::c_char,
-                ),
-                0
-            );
-            free(buf1 as *mut libc::c_void);
-            free(buf2 as *mut libc::c_void);
+        let buf1 = dc_encode_ext_header("Björn Petersen");
+        assert_eq!(&buf1, "utf-8\'\'Bj%C3%B6rn%20Petersen");
+        let buf2 = dc_decode_ext_header(buf1.as_bytes());
+        assert_eq!(&buf2, "Björn Petersen",);
 
-            buf1 = dc_decode_ext_header(
-                b"iso-8859-1\'en\'%A3%20rates\x00" as *const u8 as *const libc::c_char,
-            );
-            assert_eq!(
-                strcmp(
-                    buf1,
-                    b"\xc2\xa3 rates\x00" as *const u8 as *const libc::c_char,
-                ),
-                0
-            );
-            free(buf1 as *mut libc::c_void);
+        let buf1 = dc_decode_ext_header(b"iso-8859-1\'en\'%A3%20rates");
+        assert_eq!(buf1, "£ rates",);
 
-            buf1 = dc_decode_ext_header(b"wrong\'format\x00" as *const u8 as *const libc::c_char);
-            assert_eq!(
-                strcmp(
-                    buf1,
-                    b"wrong\'format\x00" as *const u8 as *const libc::c_char,
-                ),
-                0
-            );
-            free(buf1 as *mut libc::c_void);
+        let buf1 = dc_decode_ext_header(b"wrong\'format");
+        assert_eq!(buf1, "wrong\'format",);
 
-            buf1 = dc_decode_ext_header(b"\'\'\x00" as *const u8 as *const libc::c_char);
-            assert_eq!(
-                strcmp(buf1, b"\'\'\x00" as *const u8 as *const libc::c_char),
-                0
-            );
-            free(buf1 as *mut libc::c_void);
+        let buf1 = dc_decode_ext_header(b"\'\'");
+        assert_eq!(buf1, "\'\'");
 
-            buf1 = dc_decode_ext_header(b"x\'\'\x00" as *const u8 as *const libc::c_char);
-            assert_eq!(strcmp(buf1, b"\x00" as *const u8 as *const libc::c_char), 0);
-            free(buf1 as *mut libc::c_void);
+        let buf1 = dc_decode_ext_header(b"x\'\'");
+        assert_eq!(buf1, "");
 
-            buf1 = dc_decode_ext_header(b"\'\x00" as *const u8 as *const libc::c_char);
-            assert_eq!(
-                strcmp(buf1, b"\'\x00" as *const u8 as *const libc::c_char),
-                0
-            );
-            free(buf1 as *mut libc::c_void);
+        let buf1 = dc_decode_ext_header(b"\'");
+        assert_eq!(buf1, "\'");
 
-            buf1 = dc_decode_ext_header(b"\x00" as *const u8 as *const libc::c_char);
-            assert_eq!(strcmp(buf1, b"\x00" as *const u8 as *const libc::c_char), 0);
-            free(buf1 as *mut libc::c_void);
-        }
+        let buf1 = dc_decode_ext_header(b"");
+        assert_eq!(buf1, "");
+
+        // regressions
+        assert_eq!(
+            dc_decode_ext_header(dc_encode_ext_header("%0A").as_bytes()),
+            "%0A"
+        );
     }
 
     #[test]
     fn test_dc_needs_ext_header() {
-        unsafe {
-            assert_eq!(
-                dc_needs_ext_header(b"Bj\xc3\xb6rn\x00" as *const u8 as *const libc::c_char),
-                true
-            );
-            assert_eq!(
-                dc_needs_ext_header(b"Bjoern\x00" as *const u8 as *const libc::c_char),
-                false
-            );
-            assert_eq!(
-                dc_needs_ext_header(b"\x00" as *const u8 as *const libc::c_char),
-                false
-            );
-            assert_eq!(
-                dc_needs_ext_header(b" \x00" as *const u8 as *const libc::c_char),
-                true
-            );
-            assert_eq!(
-                dc_needs_ext_header(b"a b\x00" as *const u8 as *const libc::c_char),
-                true
-            );
-            assert_eq!(
-                dc_needs_ext_header(0 as *const u8 as *const libc::c_char),
-                false
-            );
-        }
+        assert_eq!(dc_needs_ext_header("Björn"), true);
+        assert_eq!(dc_needs_ext_header("Bjoern"), false);
+        assert_eq!(dc_needs_ext_header(""), false);
+        assert_eq!(dc_needs_ext_header(" "), true);
+        assert_eq!(dc_needs_ext_header("a b"), true);
     }
 
     #[test]
@@ -849,69 +408,20 @@ mod tests {
         assert_eq!(to_string(hex.as_ptr() as *const _), "=3A");
     }
 
-    #[test]
-    fn test_dc_urlencode_urldecode() {
-        unsafe {
-            let buf1 = percent_encode(b"Bj\xc3\xb6rn Petersen", NON_ALPHANUMERIC)
-                .to_string()
-                .strdup();
+    use proptest::prelude::*;
 
-            assert_eq!(
-                CStr::from_ptr(buf1 as *const libc::c_char)
-                    .to_str()
-                    .unwrap(),
-                "Bj%C3%B6rn%20Petersen"
-            );
-
-            let buf2 = dc_urldecode(buf1);
-
-            assert_eq!(
-                strcmp(
-                    buf2,
-                    b"Bj\xc3\xb6rn Petersen\x00" as *const u8 as *const libc::c_char
-                ),
-                0
-            );
-
-            free(buf1 as *mut libc::c_void);
-            free(buf2 as *mut libc::c_void);
+    proptest! {
+        #[test]
+        fn test_ext_header_roundtrip(buf: String) {
+            let encoded = dc_encode_ext_header(&buf);
+            let decoded = dc_decode_ext_header(encoded.as_bytes());
+            assert_eq!(buf, decoded);
         }
-    }
 
-    #[test]
-    fn test_dc_encode_decode_modified_utf7() {
-        unsafe {
-            let buf1 = dc_encode_modified_utf7(
-                b"Bj\xc3\xb6rn Petersen\x00" as *const u8 as *const libc::c_char,
-                1,
-            );
-
-            assert_eq!(
-                CStr::from_ptr(buf1 as *const libc::c_char)
-                    .to_str()
-                    .unwrap(),
-                "Bj&APY-rn_Petersen"
-            );
-
-            let buf2 = dc_decode_modified_utf7(buf1, 1);
-
-            assert_eq!(
-                strcmp(
-                    buf2,
-                    b"Bj\xc3\xb6rn Petersen\x00" as *const u8 as *const libc::c_char
-                ),
-                0
-            );
-
-            free(buf1 as *mut libc::c_void);
-            free(buf2 as *mut libc::c_void);
+        #[test]
+        fn test_ext_header_decode_anything(buf: Vec<u8>) {
+            // make sure this never panics
+            let _decoded = dc_decode_ext_header(&buf);
         }
     }
-    #[test]
-    fn test_hex_to_int() {
-        assert_eq!(hex_2_int(b'A' as libc::c_char), 10);
-        assert_eq!(hex_2_int(b'a' as libc::c_char), 10);
-        assert_eq!(hex_2_int(b'4' as libc::c_char), 4);
-        assert_eq!(hex_2_int(b'K' as libc::c_char), 20);
-    }
 }
diff --git a/src/lib.rs b/src/lib.rs
index c3a8cb079..4fb0049b6 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -16,6 +16,8 @@ extern crate rusqlite;
 extern crate strum;
 #[macro_use]
 extern crate strum_macros;
+#[macro_use]
+extern crate jetscii;
 
 #[macro_use]
 mod log;