How to get public key of a secure webpage?

This command will show you the certificate (use -showcerts as an extra parameter if you want to see the full chain):

openssl s_client -connect the.host.name:443

This will get the certificate and print out the public key:

openssl s_client -connect the.host.name:443 | openssl x509 -pubkey -noout

If you want to dig further, this question might be of interest.

In google chrome, go to the https web page (say https://mail.google.com), click on the lock next to the URL, then click on "certificate information", click on the "Details" tab, and then find "Subject Public Key Info", which for mail.google.com says Subject Public Key Algorithm: "PKCS #1 RSA Encryption" and Subject's Public Key:

Modulus (1024 bits):
AF 39 15 98 68 E4 92 FE 4F 4F F1 BB FF 0D 2E B0
FE 25 AA BD 68 04 67 27 EA 6C 43 4C A7 6D CB C8
8F 7E 81 EE 87 26 25 10 12 54 33 9E AA 3D 9B 8F
8E 92 B3 4B 01 E3 F9 4A 29 C3 0F FD AC B7 D3 4C
97 29 3F 69 55 CF 70 83 04 AF 2E 04 6E 74 D6 0F
17 09 FE 9E 20 24 24 E3 C7 68 9C AC 11 BD 92 E4
B2 1B 09 F2 02 32 BB 55 1B 2D 16 5F 30 12 23 E2
4C 4A 8D C2 DA 3F E1 B8 BF F7 3A B1 86 BE F0 C5

Public Exponent (24 bits):
01 00 01

This corresponds to the decimal numbers N=123045765072979014913511278179231402316770142733119510397469126703890433473616878571037576880489731114142173557070569117331013682110047985574967516701153320350820086344743718518661070276817382223900929884490765709232088941906671668904441902033030813416432622693650320785674286428555963210887699747381872357573

and exponent=65537 used in RSA.