TheAlgorithms/Java · error · IllegalArgumentException

Invalid Base64 character: {}

Error message

Invalid Base64 character: {}

What it means

Thrown by the private Base64.getBase64Value(char) helper when a character is outside the standard Base64 alphabet (A-Z, a-z, 0-9, '+', '/'). This is RFC 4648 standard alphabet only; URL-safe characters ('-' and '_') are not accepted, and whitespace/newlines are not skipped.

Source

Thrown at src/main/java/com/thealgorithms/conversions/Base64.java:200

     * Gets the numeric value of a Base64 character.
     *
     * @param c the Base64 character
     * @return the numeric value (0-63)
     * @throws IllegalArgumentException if character is not a valid Base64 character
     */
    private static int getBase64Value(char c) {
        if (c >= 'A' && c <= 'Z') {
            return c - 'A';
        } else if (c >= 'a' && c <= 'z') {
            return c - 'a' + 26;
        } else if (c >= '0' && c <= '9') {
            return c - '0' + 52;
        } else if (c == '+') {
            return 62;
        } else if (c == '/') {
            return 63;
        } else {
            throw new IllegalArgumentException("Invalid Base64 character: " + c);
        }
    }
}

View on GitHub (pinned to fdfb9a395b)

Solutions

1. For URL-safe input, translate '-' -> '+' and '_' -> '/', or use java.util.Base64.getUrlDecoder().
2. Strip all whitespace and newlines: input.replaceAll("\\s", "").
3. Switch to java.util.Base64.getMimeDecoder() for MIME-wrapped input with line breaks.

Example fix

// before
byte[] out = Base64.decode("a-b_c"); // URL-safe chars rejected

// after
String std = input.replace('-', '+').replace('_', '/');
while (std.length() % 4 != 0) std += "=";
byte[] out = Base64.decode(std);

String s = input.replaceAll("\\s", "").replace('-', '+').replace('_', '/');
while (s.length() % 4 != 0) s += "=";
for (char c : s.toCharArray()) {
    if (!(Character.isLetterOrDigit(c) || c == '+' || c == '/' || c == '=')) {
        throw new IllegalArgumentException("invalid char: " + c);
    }
}
byte[] out = Base64.decode(s);

static boolean isStandardBase64Alphabet(String s) {
    for (char c : s.toCharArray()) {
        if (!(Character.isLetterOrDigit(c) || c == '+' || c == '/' || c == '=')) return false;
    }
    return true;
}

try {
    byte[] out = Base64.decode(input);
} catch (IllegalArgumentException e) {
    // try URL-safe or MIME decoder
    out = java.util.Base64.getUrlDecoder().decode(input);
}

When it happens

Trigger: Passing URL-safe Base64 that uses '-' or '_' instead of '+' or '/'. Passing input with embedded whitespace, newlines, or carriage returns. Passing a character outside the 65-symbol alphabet (including padding in a data position).

Common situations: JWTs and URLs use base64url encoding which is incompatible. Base64 output wrapped at 76 columns (MIME) contains newlines. Copy-paste from sources that introduced stray characters.

Related errors

• Invalid Base64 input length; must be multiple of 4
• Padding '=' can only appear at the end (last 1 or 2 characte
• A padding '=' must not be followed by a non-padding characte
• invalid character:{}
• Input cannot be null

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/f985e20d9316b322. Report an issue: GitHub.