{
  "id": "encoding/base64",
  "family": "encoding",
  "slug": "base64",
  "title": "Base64 (RFC 4648 §4)",
  "summary": "Base64 maps every 3 input bytes to 4 ASCII characters drawn from a 64-symbol alphabet (A–Z, a–z, 0–9, '+', '/'), padding the final group with '=' so the output length is always a multiple of 4. It is the canonical binary-to-text encoding for MIME, data: URLs, PEM, HTTP Basic auth, and JWT headers/payloads (which use the URL-safe variant).",
  "kind": "encoding",
  "aliases": [
    "base-64",
    "RFC 4648 base64",
    "standard base64",
    "MIME base64"
  ],
  "status": "standard",
  "verification": "verified",
  "tier": "A",
  "source_url": "https://www.rfc-editor.org/rfc/rfc4648",
  "source_version": "RFC 4648",
  "retrieved_date": "2026-05-29",
  "see_also": [
    "encoding/base64url",
    "encoding/base32",
    "encoding/base16"
  ],
  "ext_type": "encoding@1",
  "ext": {
    "rfc": "RFC4648",
    "charset": "US-ASCII",
    "algorithm": "base64",
    "alphabet": "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
    "notes": [
      "Encodes 24 input bits (3 bytes) as four 6-bit groups; each group indexes the 64-character alphabet.",
      "Padding: '=' fills the last 4-character quantum when the input is not a multiple of 3 bytes. 1 leftover byte (8 bits) -> two chars + '=='; 2 leftover bytes (16 bits) -> three chars + '='. A 3-byte multiple needs no padding.",
      "Alphabet index 62 is '+' and 63 is '/'. The URL- and filename-safe variant (RFC 4648 §5, encoding/base64url) substitutes '-' and '_' so the output is safe in URLs and filenames.",
      "RFC 4648 §3.3: decoders MUST reject characters outside the alphabet unless explicitly told to ignore them; this entry's standard alphabet does not permit line breaks. MIME (RFC 2045) inserts CRLF every 76 characters, which is a separate, looser profile."
    ],
    "test_vectors": [
      {
        "input": "",
        "input_form": "utf8",
        "output": "",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"\") = \"\"."
      },
      {
        "input": "f",
        "input_form": "utf8",
        "output": "Zg==",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"f\") = \"Zg==\" (1 byte -> 2 chars + '==')."
      },
      {
        "input": "fo",
        "input_form": "utf8",
        "output": "Zm8=",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"fo\") = \"Zm8=\" (2 bytes -> 3 chars + '=')."
      },
      {
        "input": "foo",
        "input_form": "utf8",
        "output": "Zm9v",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"foo\") = \"Zm9v\" (3 bytes -> 4 chars, no padding)."
      },
      {
        "input": "foob",
        "input_form": "utf8",
        "output": "Zm9vYg==",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"foob\") = \"Zm9vYg==\"."
      },
      {
        "input": "fooba",
        "input_form": "utf8",
        "output": "Zm9vYmE=",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"fooba\") = \"Zm9vYmE=\"."
      },
      {
        "input": "foobar",
        "input_form": "utf8",
        "output": "Zm9vYmFy",
        "output_form": "ascii",
        "algorithm": "base64",
        "direction": "encode",
        "note": "RFC 4648 §10 test vector: BASE64(\"foobar\") = \"Zm9vYmFy\" (6 bytes -> 8 chars, no padding)."
      },
      {
        "input": "Zm9vYmFy",
        "input_form": "ascii",
        "output": "foobar",
        "output_form": "utf8",
        "algorithm": "base64",
        "direction": "decode",
        "note": "Round-trip: BASE64-decode(\"Zm9vYmFy\") = \"foobar\"."
      }
    ]
  },
  "updated": "2026-05-29T00:00:00Z"
}