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I was investigating the Client Hello for my iOS app and saw that the TLS 1.3 handshake with Client Hello sends Signature Algorithm: rsa_pkcs1_sha1 (0x0201) Signature Hash Algorithm Hash: SHA1 (2) Signature Hash Algorithm Signature: RSA (1) I thought SHA-1 is not being used anymore. The Full list of offered signature_algorithms from the client in the Extension: signature_algorithms (len=24) Type: signature_algorithms (13) Length: 24 Signature Hash Algorithms Length: 22 Signature Hash Algorithms (11 algorithms) Signature Algorithm: ecdsa_secp256r1_sha256 (0x0403) Signature Hash Algorithm Hash: SHA256 (4) Signature Hash Algorithm Signature: ECDSA (3) Signature Algorithm: rsa_pss_rsae_sha256 (0x0804) Signature Hash Algorithm Hash: Unknown (8) Signature Hash Algorithm Signature: SM2 (4) Signature Algorithm: rsa_pkcs1_sha256 (0x0401) Signature Hash Algorithm Hash: SHA256 (4) Signature Hash Algorithm Signature: RSA (1) Signature Algorithm: ecdsa_secp384r1_sha384 (0x0503) Signature Hash Algorithm Hash: SHA384 (5) Signature Hash Algorithm Signature: ECDSA (3) Signature Algorithm: ecdsa_sha1 (0x0203) Signature Hash Algorithm Hash: SHA1 (2) Signature Hash Algorithm Signature: ECDSA (3) Signature Algorithm: rsa_pss_rsae_sha384 (0x0805) Signature Hash Algorithm Hash: Unknown (8) Signature Hash Algorithm Signature: Unknown (5) Signature Algorithm: rsa_pss_rsae_sha384 (0x0805) Signature Hash Algorithm Hash: Unknown (8) Signature Hash Algorithm Signature: Unknown (5) Signature Algorithm: rsa_pkcs1_sha384 (0x0501) Signature Hash Algorithm Hash: SHA384 (5) Signature Hash Algorithm Signature: RSA (1) Signature Algorithm: rsa_pss_rsae_sha512 (0x0806) Signature Hash Algorithm Hash: Unknown (8) Signature Hash Algorithm Signature: Unknown (6) Signature Algorithm: rsa_pkcs1_sha512 (0x0601) Signature Hash Algorithm Hash: SHA512 (6) Signature Hash Algorithm Signature: RSA (1) Signature Algorithm: rsa_pkcs1_sha1 (0x0201) Signature Hash Algorithm Hash: SHA1 (2) Signature Hash Algorithm Signature: RSA (1)