RFC 1751 S/KEY word encoder

RFC 1751 defines a fixed convention for turning binary cryptographic keys into short strings of English words so people can read, remember, and type 64-bit and 128-bit material without hex fatigue. The same dictionary powers classic S/KEY one-time passwords: each eight-byte block becomes six words drawn from a 2048-word list of one-to-four-letter tokens, with two spare bits reserved for parity so transcription mistakes are caught on the way back. This capability encodes a hexadecimal key into that word form and decodes words back to uppercase hex, verifying the parity bits on every six-word group.

How RFC 1751 packs 64 bits into six English words

The RFC 1751 dictionary contains exactly two thousand forty-eight English words of one to four letters, so each word indexes eleven bits. Six words supply sixty-six bits: sixty-four bits of key material plus a two-bit parity field. Encoding walks each eight-byte half of your key as a bit stream, sums consecutive two-bit slices over the sixty-four data bits to form a parity nibble, appends the low two parity bits, then slices the sixty-six-bit result into six eleven-bit indices.

Those indices select words from the fixed list in uppercase, joined by spaces. A pure sixty-four-bit key therefore becomes exactly six words; a one-hundred-twenty-eight-bit key is two independent halves and becomes twelve words. Decoding reverses the map: each word must appear in the dictionary, six words rebuild nine bytes of bit packing, the first eight bytes form the candidate key half, and the stored two parity bits are compared against a fresh parity sum.

A mismatch means a mistyped word or a bit flip and is rejected with a clear parity-failure message rather than a silent wrong key. The dictionary never changes across implementations; that stability is why S/KEY OTPs, SNMP-era key printouts, and modern recovery tools can interoperate when they all claim RFC 1751. This page implements that classic word list only—not BIP-39 mnemonics, not EFF diceware, and not arbitrary passphrase generators—so search intent stays locked to human-readable key encoding with parity.

Where S/KEY-style word keys still earn their keep

Operators who still print or dictate sixty-four-bit material benefit from six short words far more than from sixteen hex digits, especially over a phone or a radio channel. S/KEY and OTP systems that already emit RFC 1751 phrases need a strict decoder that rejects parity failures instead of accepting a neighbor key that would authenticate the wrong principal. Security training labs teach transcription hygiene by showing how a single wrong word flips the parity bit and aborts decode.

Incident response notes sometimes store offline recovery material as human-readable word groups so a responder without a hex editor can still re-enter a key later. Documentation authors ranking for RFC 1751 or S/KEY words want a dedicated landing with a live tool rather than a generic Base32 or BIP-39 page, because the dictionary, parity rule, and key lengths differ. Failed validation does not consume balance.

Keep requests to the documented key sizes, pin the uppercase hex and word strings in tests, and you get reproducible transforms for recovery workflows, OTP tooling, and edge workers without standing up a full cryptography toolkit of your own.