What is the ASC file format?
An .asc file contains a set of ASCII-encoded characters representing an OpenPGP-armored payload - a public key, encrypted message, or detached signature that has been encoded into printable text for safe transmission. It can be readable text at a glance (the header lines) as well as cryptographic data unreadable to humans in its raw binary form, here wrapped in Base64 so it survives email clients and plain-text pipelines intact.
American Standard Code for Information Interchange (ASCII) is a character encoding standard that uses 7-bit code and can encode up to 128 characters or instructions. Each number from 0 to 127 encodes a symbol or character, from 95 of which are printable characters. Those are:
- A-Z: upper-case letters of the Latin alphabet,
- a-z: lower-case letters of the Latin alphabet,
- 0-9: digits,
- Operators and brackets,
- Punctuation marks,
- Special characters.
Most control characters are outdated as they were used by teleprinters. Most modern encoding systems are derived from the ASCII system, including UTF-8, which became the dominant encoding for the internet.
.asc does not have the capability to encode diacritical or special characters beyond those used in basic American English, but that is by design - the binary cryptographic payload is wrapped in Base64 (radix-64), which uses only safe printable ASCII characters. OpenPGP's armor format goes back to PGP's origins in the early 1990s and was standardized in RFC 4880, with the latest revision in RFC 9580. .asc files are still widely used for distributing GPG keys, software release signatures, encrypted messages, and signed email content. Every .asc file is recognizable by its opening line - -----BEGIN PGP PUBLIC KEY BLOCK-----, -----BEGIN PGP MESSAGE-----, or -----BEGIN PGP SIGNATURE----- - and a matching -----END PGP ...----- footer.
Security & safety
RISK: LOWAn OpenPGP-armored .asc is plain text and cannot execute - opening one to read it is safe. The real security points are about trust, not malware: when you VERIFY a download with a .asc detached signature, a 'Good signature' only means anything if you trust the public key - confirm the key's fingerprint from the vendor's official site, not from the same place you got the file. A public-key .asc is harmless to import but lets others be impersonated if you trust the wrong key. Never paste a private-key .asc or its passphrase into a website; decrypt an encrypted .asc locally with GnuPG rather than an online tool.
Format details
in a nutshell- Generic ASCII text file - Historically .asc simply meant a plain ASCII text file readable in any text editor - the legacy meaning with no PGP header.
- Autodesk / CAD ASCII export - Some CAD/survey tools export .asc as plain-text XYZ coordinate lists, ESRI ASCII-grid rasters, or point clouds for re-import into CAD/GIS software.
Programs that open ASC files
Windows2 apps
Gpg4win (Kleopatra)
Open-source Drag the .asc into Kleopatra (or right-click → More GpgEX options → Verify/Decrypt). To add a key, File → Import Certificates.
Notepad / Notepad++
Open-source Open as text to read the content and confirm the '-----BEGIN PGP...' header (tells you which OpenPGP block it is). 
macOS2 apps
TextEdit
Built-in Open as plain text to inspect the armor header; use Format → Make Plain Text to avoid RTF. 
Linux2 apps
GnuPG (gpg)
Open-source Verify: 'gpg --verify file.asc file'; import a key: 'gpg --import key.asc'; decrypt: 'gpg --decrypt msg.asc'.
Seahorse / gedit
Open-source Seahorse imports keys and verifies/decrypts from the file manager; any text editor opens the armor as text. Technical details
deep spec| Format type | Plain-text OpenPGP armor; binary cryptographic payload encoded in Base64 (radix-64) |
| Container types | Public key block, encrypted message, detached signature, or clearsigned message |
| Header and footer delimiters | -----BEGIN PGP <TYPE>----- and -----END PGP <TYPE>----- lines (types include PUBLIC KEY BLOCK, MESSAGE, SIGNATURE, SIGNED MESSAGE) |
| Payload encoding | Base64 (radix-64, 64-character alphabet with = padding), line-wrapped at 76 characters |
| Integrity check | CRC-24 checksum embedded as a =-prefixed Base64 line immediately before the closing footer |
| Armor metadata headers | Optional plaintext key-value headers between the armor header and the Base64 body (e.g. Hash:, Version:, Comment:) |
| MIME types | application/pgp-signature (signature), application/pgp-keys (key), application/pgp-encrypted (message) |
| Algorithm support | Algorithm-agnostic; supports RSA, DSA, ElGamal, ECDSA, Ed25519, Ed448, X25519, and X448 |
| Compression | Binary payload may be ZLIB- or ZIP-compressed before Base64 encoding |
| Text encoding | 7-bit ASCII only; safe for all email systems, plain-text protocols, and source-code repositories |
| Related binary formats | .gpg and .pgp carry the same OpenPGP data in raw binary; .sig is used for detached binary signatures |
| Key distribution | Standard format for publishing OpenPGP public keys on keyservers and in KEYS files for software projects |
| Git integration | Used for GPG-signed Git commits and tags; verified with the gpg --verify command |
| Cleartext signing | Supports the SIGNED MESSAGE armor type, which signs human-readable text while keeping it readable without decryption |
| Released | OpenPGP armor since PGP, 1991; standardized in RFC 4880 (2007) / RFC 9580 (2024) |
| Open standard | Yes · royalty-free |
| Specification | www.rfc-editor.org |
ASC conversions
Community Q&A
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