java.lang.NoClassDefFoundError: org/bouncycastle/asn1/ASN1Encodable

I use iText 5.5.0 and BouncyCastle bcprov-jdk15on-1.50 for digitally signing a pdf. It all works fine if I do not include a time stamp in the MakeSignature.signDetached method.

When I use the TSAClient specifying the TSAURL, my code gives me the error

java.lang.NoClassDefFoundError: org/bouncycastle/asn1/ASN1Encodable at MakeSignature.signDetached method

and my program gets terminated even though I handled exceptions.

I tried different versions of bouncycastle jars but I get the same error with difference in the classes not found.

I really need Timestamp included in the Signature. What should I do.