Puttygen cer pem

I have zero knowledge on certificates . I searched on internet and found few, but none of them helped me in my case.

I am on Red Hat Enterprise Linux ES release 4 (Nahant Update 9)

I am trying to convert .cer ( this is a CAS certificate) to .pem and getting the below error

openssl x509 -inform der -in cas.cer -out cas.pem unable to load certificate 4419:error:0D0680A8:asn1 encoding routines:ASN1_CHECK_TLEN:wrong tag:tasn_dec.c:959: 4419:error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error:tasn_dec.c:304:Type=X509 I am able to read the plain text of it just by running

Puttygen rsa 4096

Puttygen aws key

Puttygen heise

Puttygen pem

Puttygen.exe help