TheAlgorithms/Java · error · IllegalArgumentException

A padding '=' must not be followed by a non-padding characte

Error message

A padding '=' must not be followed by a non-padding character

What it means

Base64.decode rejects input where a '=' padding character is followed by a non-'=' character (e.g., "AB=CD"). RFC 4648 requires that once padding begins, all subsequent characters must also be padding. This catches strings where padding appears, then a data character follows, indicating corruption.

Source

Thrown at src/main/java/com/thealgorithms/conversions/Base64.java:128

        if (input.isEmpty()) {
            return new byte[0];
        }

        // Strict RFC 4648 compliance: length must be a multiple of 4
        if (input.length() % 4 != 0) {
            throw new IllegalArgumentException("Invalid Base64 input length; must be multiple of 4");
        }

        // Validate padding: '=' can only appear at the end (last 1 or 2 chars)
        int firstPadding = input.indexOf('=');
        if (firstPadding != -1) {
            if (firstPadding < input.length() - 2) {
                throw new IllegalArgumentException("Padding '=' can only appear at the end (last 1 or 2 characters)");
            }
            for (int i = firstPadding; i < input.length(); i++) {
                if (input.charAt(i) != '=') {
                    throw new IllegalArgumentException("A padding '=' must not be followed by a non-padding character");
                }
            }
        }

        List<Byte> result = new ArrayList<>();

        // Process input in groups of 4 characters
        for (int i = 0; i < input.length(); i += 4) {
            // Get up to 4 characters
            int char1 = getBase64Value(input.charAt(i));
            int char2 = getBase64Value(input.charAt(i + 1));
            int char3 = input.charAt(i + 2) == '=' ? 0 : getBase64Value(input.charAt(i + 2));
            int char4 = input.charAt(i + 3) == '=' ? 0 : getBase64Value(input.charAt(i + 3));

            // Combine four 6-bit groups into a 24-bit number
            int combined = (char1 << 18) | (char2 << 12) | (char3 << 6) | char4;

            // Extract three 8-bit bytes

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Strip all whitespace and newlines before decoding: input = input.replaceAll("\\s", "").
2. Re-encode the source data as a single Base64 string instead of concatenating fragments.
3. Validate with a regex like ^[A-Za-z0-9+/]*={0,2}$ before calling decode.

Example fix

// before
String joined = frag1 + frag2; // frag1 ended with '=', frag2 starts with data
byte[] out = Base64.decode(joined);

// after
byte[] raw = (decodeOrFetch(frag1) + decodeOrFetch(frag2)).getBytes();
String joined = Base64.encode(raw);

String s = input.replaceAll("\\s", "");
int fp = s.indexOf('=');
if (fp != -1) {
    for (int i = fp; i < s.length(); i++) {
        if (s.charAt(i) != '=') throw new IllegalArgumentException("non-pad after pad");
    }
}
byte[] out = Base64.decode(s);

static boolean paddingIsContiguousAtEnd(String s) {
    s = s.replaceAll("\\s", "");
    int fp = s.indexOf('=');
    if (fp == -1) return true;
    for (int i = fp; i < s.length(); i++) if (s.charAt(i) != '=') return false;
    return true;
}

try {
    byte[] out = Base64.decode(input);
} catch (IllegalArgumentException e) {
    throw new DomainException("Invalid Base64 padding: " + e.getMessage(), e);
}

When it happens

Trigger: Input like "AB=C=" or "A=BC" where '=' is followed by a non-'=' character. Strings assembled from fragments where padding leaked into the middle. Base64 strings with trailing whitespace after padding that wasn't stripped.

Common situations: Concatenating two Base64 fragments without re-encoding. Log lines or wrapped text where '=' from one chunk precedes data from the next. Whitespace/newline characters appearing after padding.

Related errors

• Padding '=' can only appear at the end (last 1 or 2 characte
• Invalid Base64 input length; must be multiple of 4
• Input cannot be null
• Invalid Base64 character: {}
• Slope and intercept must be valid numbers.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/641c10639edf37dc. Report an issue: GitHub.