Generate key default

生成される認証用秘密鍵の種類と長さのデフォルト

software	cihper	length	date
OpenSSH -4.1	RSA	1024
OpenSSH 4.2-	RSA	2048	2005-09-01
OpenSSH 8.0-	RSA	3072	2019-04-17
OpenSSH 9.5-	Ed25519	256	2023-10-04
PuTTYgen -0.62	RSA	1024
PuTTYgen 0.63-	RSA	2048	2013-08-06
RLogin -2.25.2	RSA	2048
RLogin 2.25.3-	RSA	3072	2020-06-22
RLogin 2.26.5-	ECDSA	256	2021-11-29
Tera Term 4.62-	RSA	2048	2009-03-22
Tera Term 5.0-	Ed25519	256	2023-10-15

2005-06-08 http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-keygen.c#rev1.127
2005-09-01 OpenSSH 4.2 https://www.openssh.com/txt/release-4.2
  - Increase the default size of new RSA/DSA keys generated by
    ssh-keygen from 1024 to 2048 bits.

2019-03-25 http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-keygen.c#rev1.328
2019-04-17 OpenSSH 8.0 https://www.openssh.com/txt/release-8.0
 * ssh-keygen(1): Increase the default RSA key size to 3072 bits,
   following NIST Special Publication 800-57's guidance for a
   128-bit equivalent symmetric security level.

2023-09-04 http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/ssh-keygen.c#rev1.471
2023-10-04 OpenSSH 9.5 https://www.openssh.com/txt/release-9.5
 * ssh-keygen(1): generate Ed25519 keys by default. Ed25519 public keys
   are very convenient due to their small size. Ed25519 keys are
   specified in RFC 8709 and OpenSSH has supported them since version 6.5
   (January 2014).