File extension

.asc

ASCII-armored OpenPGP data

.asc files are most often ASCII-armored OpenPGP data (signatures, public keys, or encrypted messages in text form), but the extension is also used for plain ASCII text.

Known meanings

Formats and reference material for .asc

ASCII-armored OpenPGP data

Text encoding of OpenPGP signatures, keys, and encrypted messages

  • data-format internet
  • IETF OpenPGP Working Group
Open link

ASCII text

Plain text restricted to the ASCII character set

  • documents
  • IETF / ANSI
Open link

Details

Format notes

ASC (ASCII-Armored OpenPGP)

The most common use of .asc is ASCII armor, the text form of OpenPGP data. It lets binary data that would otherwise be stored as .gpg or .pgp pass through text-only channels such as e-mail and web pages. Software projects usually publish a detached signature next to a download, such as release.tar.gz.asc, and people share public keys as .asc files.

Structure

ASCII armor, specified in OpenPGP (RFC 4880 and RFC 9580), wraps the Base64 encoding of the binary packets between header and footer lines that show the content type, for example -----BEGIN PGP PUBLIC KEY BLOCK-----, -----BEGIN PGP SIGNATURE-----, or -----BEGIN PGP MESSAGE-----. Optional header lines such as Version: or Comment: can follow, and the encoded data may be followed by a CRC-24 checksum line starting with =. A clear-signed message (-----BEGIN PGP SIGNED MESSAGE-----) keeps the text readable and puts the signature after it. Armor makes the data about a third larger than binary.

Other uses

Outside OpenPGP, .asc can simply denote a plain ASCII text file, as in some scientific and geographic data exports (for example, grids written as text). Check the content: armor files start with -----BEGIN PGP.

Preservation And Security Notes

A signature file is only useful together with the exact file it signs and the signer’s public key, so archive these together. Verify keys by fingerprint rather than trusting a key downloaded from the same place as the file. Armored files marked as private keys must be protected like passwords.

Further Reading

  • RFC 9580, OpenPGP: https://www.rfc-editor.org/rfc/rfc9580
  • RFC 4880, OpenPGP Message Format (Section 6, Radix-64 Conversions): https://www.rfc-editor.org/rfc/rfc4880
  • RFC 20, ASCII format for network interchange: https://www.rfc-editor.org/rfc/rfc20