rsa_pkcs1_sha1
rsa_pkcs1_sha1 is a legacy RSA signature scheme with SHA-1, deprecated for TLS. Only a few measured clients still offer it.
What it is
rsa_pkcs1_sha1 (code 0x0201) is RSA PKCS#1 v1.5 with SHA-1. In TLS 1.3, RFC 8446 lists it only as a legacy value for verifying old certificates, never for handshake signatures. RFC 9155 later deprecated SHA-1 signatures in TLS 1.2 as well.
SHA-1 is no longer collision resistant, which can be used to forge signatures in some settings; RFC 9155 gives this as the reason for deprecating it. Publicly trusted certificate authorities stopped issuing SHA-1 certificates years ago.
Clients that still list it do so for compatibility with old servers or private PKI that never migrated. A server should not rely on it.
Removing it from a server configuration is safe for modern visitors, since every client we recorded also offers SHA-256 based schemes.
Who sends it
Measured on 2026-10-25 against our own test server: sent by 3 of 11 clients. Position counts from 1 and ignores GREASE values.
Why it matters for fingerprinting
In our measurements only the Firefox profile of curl-impersonate, macOS curl and Go net/http offer it. Python requests, httpx, Node.js fetch, Guzzle, Debian curl, curl_cffi and curl-impersonate chrome116 do not. Our Chrome capture did not record signature algorithms.
Check whether your client sends it
Open the test from your browser, or call it from your script, and look for rsa_pkcs1_sha1 in the TLS details.
Run the TLS fingerprint test