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    //! Base64 encoding support.
    use cvt_n;
    use error::ErrorStack;
    use ffi;
    use libc::c_int;

    /// Encodes a slice of bytes to a base64 string.
    ///
    /// This corresponds to [`EVP_EncodeBlock`].
    ///
    /// # Panics
    ///
    /// Panics if the input length or computed output length overflow a signed C integer.
    ///
    /// [`EVP_EncodeBlock`]: https://www.openssl.org/docs/man1.1.1/man3/EVP_DecodeBlock.html
    pub fn encode_block(src: &[u8]) -> String {
        assert!(src.len() <= c_int::max_value() as usize);
        let src_len = src.len() as c_int;

        let len = encoded_len(src_len).unwrap();
        let mut out = Vec::with_capacity(len as usize);

        // SAFETY: `encoded_len` ensures space for 4 output characters
        // for every 3 input bytes including padding and nul terminator.
        // `EVP_EncodeBlock` will write only single byte ASCII characters.
        // `EVP_EncodeBlock` will only write to not read from `out`.
        unsafe {
            let out_len = ffi::EVP_EncodeBlock(out.as_mut_ptr(), src.as_ptr(), src_len);
            out.set_len(out_len as usize);
            String::from_utf8_unchecked(out)
        }
    }

    /// Decodes a base64-encoded string to bytes.
    ///
    /// This corresponds to [`EVP_DecodeBlock`].
    ///
    /// # Panics
    ///
    /// Panics if the input length or computed output length overflow a signed C integer.
    ///
    /// [`EVP_DecodeBlock`]: https://www.openssl.org/docs/man1.1.1/man3/EVP_DecodeBlock.html
    pub fn decode_block(src: &str) -> Result<Vec<u8>, ErrorStack> {
        let src = src.trim();

        // https://github.com/openssl/openssl/issues/12143
        if src.is_empty() {
            return Ok(vec![]);
        }

        assert!(src.len() <= c_int::max_value() as usize);
        let src_len = src.len() as c_int;

        let len = decoded_len(src_len).unwrap();
        let mut out = Vec::with_capacity(len as usize);

        // SAFETY: `decoded_len` ensures space for 3 output bytes
        // for every 4 input characters including padding.
        // `EVP_DecodeBlock` can write fewer bytes after stripping
        // leading and trailing whitespace, but never more.
        // `EVP_DecodeBlock` will only write to not read from `out`.
        unsafe {
            let out_len = cvt_n(ffi::EVP_DecodeBlock(
                out.as_mut_ptr(),
                src.as_ptr(),
                src_len,
            ))?;
            out.set_len(out_len as usize);
        }

        if src.ends_with('=') {
            out.pop();
            if src.ends_with("==") {
                out.pop();
            }
        }

        Ok(out)
    }

    fn encoded_len(src_len: c_int) -> Option<c_int> {
        let mut len = (src_len / 3).checked_mul(4)?;

        if src_len % 3 != 0 {
            len = len.checked_add(4)?;
        }

        len = len.checked_add(1)?;

        Some(len)
    }

    fn decoded_len(src_len: c_int) -> Option<c_int> {
        let mut len = (src_len / 4).checked_mul(3)?;

        if src_len % 4 != 0 {
            len = len.checked_add(3)?;
        }

        Some(len)
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn test_encode_block() {
            assert_eq!("".to_string(), encode_block(b""));
            assert_eq!("Zg==".to_string(), encode_block(b"f"));
            assert_eq!("Zm8=".to_string(), encode_block(b"fo"));
            assert_eq!("Zm9v".to_string(), encode_block(b"foo"));
            assert_eq!("Zm9vYg==".to_string(), encode_block(b"foob"));
            assert_eq!("Zm9vYmE=".to_string(), encode_block(b"fooba"));
            assert_eq!("Zm9vYmFy".to_string(), encode_block(b"foobar"));
        }

        #[test]
        fn test_decode_block() {
            assert_eq!(b"".to_vec(), decode_block("").unwrap());
            assert_eq!(b"f".to_vec(), decode_block("Zg==").unwrap());
            assert_eq!(b"fo".to_vec(), decode_block("Zm8=").unwrap());
            assert_eq!(b"foo".to_vec(), decode_block("Zm9v").unwrap());
            assert_eq!(b"foob".to_vec(), decode_block("Zm9vYg==").unwrap());
            assert_eq!(b"fooba".to_vec(), decode_block("Zm9vYmE=").unwrap());
            assert_eq!(b"foobar".to_vec(), decode_block("Zm9vYmFy").unwrap());
        }

        #[test]
        fn test_strip_whitespace() {
            assert_eq!(b"foobar".to_vec(), decode_block(" Zm9vYmFy\n").unwrap());
            assert_eq!(b"foob".to_vec(), decode_block(" Zm9vYg==\n").unwrap());
        }
    }